The production of hydrocarbons such as oil and gas is important for both economic and strategic reasons. The recovery of oil or gas from a particular reservoir is determined by the economics of operation. If the revenue is less than the operating costs, then the well will be either shut in (to wait for higher prices, better production technology, etc.) or the well may be plugged with concrete and abandoned. To maximize cash flow and profits it is common to try to produce as much oil as possible, without damaging the well.
One known problem which can restrict the flow of oils from a well is the deposition of wax in the flow paths causing restrictions and choking off flow. As waxy crudes cool down, during transport through piping or other means from the reservoir to the refinery, paraffin wax will precipitate. The precipitated paraffins cause numerous problems such as flow obstruction due to the deposition of plugging deposits in pipelines, poor pumpability due to high viscosity and difficulty breaking water/oil emulsions that may inadvertently form during production.
In some environments such as offshore pipelines and tubulars each incident of plugging due to wax deposition can cost millions of dollars. It has been recently reported that 17 such incidents occurred in the Gulf of Mexico during a one year period. The economic incentives to prevent plugging depositions are high.
To date there is no reliable technique to predict rates of wax deposition. Lab measurements of deposition rates seldom correlate to measured rates in the field. The factors which control the solubility of the paraffin in crude are fairly straightforward to measure analytically and model with thermodynamics. However, wax deposition and plugging is usually less severe in highly waxy crudes and more severe in less waxy crudes. There are pipelines which carry extremely waxy crude and do not require any wax deposit removal through use of for example "pigging" or chemical solvent flushing/cleaning.
Similarly, measurement of waxy crude rheology (viscosity and yield strength) is not very reliable. Rheology data measured by different labs can differ by 10 fold for the same oil sample. The simulation (or lack of) "temperature/time/shear history" (that is the exact combination of events that a crude may be subjected to during production from the reservoir and transportation to a refinery) is usually used to explain the lack of consistent results between field measurements and lab measurements. Yield strengths may vary by more than 1000 fold depending on what pretreatment conditions the crude or produced hydro carbons may be subjected to.
Why are the waxy crude properties so difficult to measure reliably? Some reasons include:
1) The dominant mechanism for wax precipitation appears to be formation of suspended solids. The suspended solids are fairly innocuous and are carried along in the oil as a slurry. Typically only a small proportion (i.e., perhaps less than 1%) of the precipitated wax forms adherent deposits which can cause flow blockages or restrictions. Thus, paraffin precipitation occurs via two competing pathways, ire., suspended solids vs adherent deposits. PA1 2) The factors which control the proportion of adherent wax vs. suspended solids are more complex. For example, if there is an inventory of suspended waxy solids then these solids can absorb paraffin supersaturations due to their relatively large surface areas (see Canadian patent 1,289,497 Nenniger, J. Process for Inhibiting Formation of Wax Deposits). For example, 100 ppm of waxy solids (1 micron in diameter) in crude oil have 10 times more surface area than the inside wall of the 27/8" tubing in which they are transported. Thus, the suspended solids inhibit the formation of adherent deposits. Therefore the proportion of wax precipitating to form adherent deposits is a nonlinear function of the inventory of suspended solids. PA1 3) In lab scale simulations, it is not possible to simulate all large scale phenomena simultaneously. For example, if we match the shear rate in the oil in both the lab and the field, then the Reynold's number cannot be matched at the same time (unless the lab apparatus is dimensionally identical to the field). PA1 a chamber to hold a fluid sample; PA1 a means to provide controlled shear rates in the fluid within said chamber; PA1 a means to provide cooling/heating to the fluid within said chamber; PA1 a means to regulate the temperature gradients/heat flux in the fluid within said chamber; PA1 a means to regulate the pressure/composition of the fluid within said chamber; PA1 a means to introduce particles into the fluid within said chamber; PA1 a means to measure deposit accumulation rates on the surfaces of said chamber; PA1 a means to withdraw fluid samples from said chamber; PA1 a means to measure fluid rheology within said chamber; PA1 a means to characterize the particles the fluid within said chamber including probes and sensors p1 a means for numerical calculation of said characteristics using said sensors and probes comprising external computational devices and software PA1 a means for control of said temperature, heat flux, shear rates, pressure/composition in said chamber using external signal processing devices, computational devices and software.
To date there has not been any systematic attempt to characterize waxy solids other than to note that they are present. Thus to date, research efforts have only examined part of the deposition mechanism so prediction based on lab scale equipment has not been reliable.